Electrical – Will this Optocoupler and MOSFET be able to deliver 5V +2A

microcontrollermosfetopto-isolator

I'm working on controlling the supply of an amplifier board with 5V 2.4A power in a manner which tries to eliminate noise from the speaker. I asked a few professors at my university and they said that an optocoupler would be the best way. Since I need a higher amperage, I also need a MOSFET or transistor or some sort.

The following circuit is the result of my research and testing in Livewire. The idea is that this will then be my first custom PCB which we'll etch in the lab and then incorporate into my refurbishing of an old vintage radio.

Circuit Diagram

Q1: IRL520 (or similar) MOSFET

Q2: 4N35 Optocoupler

CN1: Line to 5V 2.4A power supply

CN2: Cables to On/Off switch

CN3: Power line to amplifier, needs at least 5V 2A

CN4: Control lines to 3.3V microcontroller, (Rasp0W or Arduino)

R1: 1k Ohms to make sure the gate isn't triggered unless microcontroller is HIGH

R2: ~250 Ohms to make sure the optocoupler's LED (Vf~=1.5V) only gets 10mA off a 3.3V line.

When I simulate the circuit by:

  • adding a 5V source at CN1

  • shorting the terminals in CN2

  • adding a lamp/led at CN3

  • adding a 3.3V source at CN4

The circuit remains closed when the microcontroller is LOW, and when it is pulled high, the voltage to the gate of the MOSFET reads 3.3V (wire marked with purple). However, it doesn't indicate that the light bulb at CN3 lights up.

Simulated Circuit Diagram

My line of thinking is that Livewire doesn't know that I'm using a logic-level MOSFET, therefore thinks that I need at least 5V to trigger the gate.

Will this circuit be able to deliver 5V 2.4A to the amplifer at CN3? Are the parts adequate or are there better options out there? Thanks!

Best Answer

This will not work. You need a p-channel MOSFET (logic-level) to switch the high side with low drop. Your circuit will drop at Vgs(on) at some current that depends on the load. Also you should increase R3 considerably since your phototransistor is barely turning on. Try 10K or 20K (or, much better, work it out from the datasheet worst-case values since you are presumably an engineering student).

schematic

simulate this circuit – Schematic created using CircuitLab

You may be able to get rid of the optocoupler and just use an ordinary NPN BJT such as a 2N4401 since the control signal is now ground-referenced.

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